In a CMOS image sensor, a two-step single-slope type ADC could be used to realize an increase in speed of a single-slope type ADC that converts an analog signal received from a pixel into a digital signal. In the two-step single-slope type ADC, an about 7.6 times increase in speed is attained. When it is attempted to attain a further increase in speed of operation of the CMOS image sensor, the increase in speed is limited by the responsiveness of a vertical signal line that transmits a signal read out from a pixel.